The present invention relates to optical communication links and more particularly to wavelength division multiplexing (WDM) links.
WDM optical communication links have evolved to incorporate the capacity to transmit optical signals over very large distances, e.g., greater than 1000 km without regeneration of the optical signal while carrying more than 200 channels at 10 Gbps. Such systems are referred to as ultra long haul (ULH) dense WDM (DWDM) systems.
It is desirable to implement a ULH link as a part of a hybrid system where certain wavelengths are configured for ULH operation while other wavelengths that are added and/or dropped along the way need merely achieve a reach of e.g., 500-700 kms. Thus so-called long haul (LH) and ULH are transmitted on the same fiber but on different wavelengths.
Due to the shorter reach of the LH wavelengths, it is not necessary to provide the same type of amplification resources as are needed for ULH operation. While the ULH link typically employs Raman amplification in addition to Erbium-doped fiber amplifiers (EDFAs) at each of a series of amplification sites along the link, the LH system generally requires only EDFA amplification technology to achieve the needed performance.
Current hybrid LH/ULH systems assign different transmission bands to LH and ULH operation. For example, the “C” band could be assigned to LH while “L” band would be assigned to ULH or vice versa. Separate C band and L band EDFAs are provided. To implement this type of system it is necessary to demultiplex and then remultiplex the two bands at each optical amplification site along the link to accommodate the separate single-band amplifiers while avoiding crosstalk between the bands. Interferential filters are typically used to provide the necessary demultiplexing and remultiplexing due to their high isolation, large transmission bandwidth, and low insertion loss.
A problem arises, however, in the use of optical interferential filters for this purpose. There will typically be amplitude ripple in the passband of the filter response. If a link includes 20 intermediate amplification sites where multiplexing and demultiplexing operations need be performed, there may be more than forty filters used for this purpose. If the filter ripple of the numerous filters is fairly correlated among the multiple filters, the overall link frequency response may be subject to relatively large peaks and nulls. This can cause certain wavelengths to fail to meet signal to noise ratio requirements for correct receiver operation. Given that the interferential filters installed along a link will typically be produced by the same manufacturer and be of the same model, a close correlation of ripple peaks and ripple nulls in the frequency domain can be expected. There is no way to readily compensate for the resulting accumulation of ripple along the link.
What is needed is a configuration for hybrid LH/ULH operation that avoids the frequency response ripple introduced by interferential filters used for multiplexing and demultiplexing along the link.